1. Field of the Invention
The present invention relates to a robot operation planning system for planning an operation in which a robot grasps and moves an object which may be placed in various three-dimensional environments.
2. Description of the Related Art
There is known a robot operation planning system for automatically determining a plan of action in which a robot performs the grasping, movement and pick-and-place operations of an object between its initial state and each of various goal states such as during an assembly process and others.
One of the robot operation plans is a grasp plan. In the grasp plan, it is probably preferred that from the viewpoint of easy operation during the approach to a grasp orientation or after being grasped, an object manipulated by the robot is situated as far away from surrounding obstacles as possible.
We have proposed a grasp planning method which utilizes space characterization near the grasping position such that an object can be grasped by the robot from a widely opened space both the pick and place operations (THE FIFTH ANNUAL MEETING OF ROBOTICS SOCIETY OF JAPAN, November 1987).
In our proposal, we perform a space characterization around an object in each of its initial and goal states. Such a space characterization is accomplished by dividing the space around the object into triangular pyramids (space pyramids) and investigating interferences between all the space pyramids and surrounding obstacles. More particularly, we determine space pyramids each having a first distance (depth) between its apex (grasp position) and an obstacle around that pyramid, this depth being equal to or larger than the distance required for the gripping portion (hand) of a robot to exist and also for the hand to be able to approach the object, that is, open space pyramids.
Next, we determine each of these open space pyramids both in the initial and goal states and select open space pyramids common to both the initial and goal states, that is, common open space pyramids. In order to investigate the distribution of these common open space pyramids, distance transformation is carried out for all the common open space pyramids. This distance transformation is accomplished by determining the minimum distance between each of the common open space pyramids and the relevant open space pyramid not selected as a common open space pyramid. Open space pyramids having larger distance transformation values indicate a widely opened space which is more or less unoccupied by obstacles.
A grasp position is thus determined by selecting a common open space pyramid having the maximum distance transformation value from the open space pyramids intersecting a central grasp plane formed between two parallel planes on the object, while considering the stable grasping of the object by two parallel fingers.
Our proposal mentioned above can grasp an object from a direction in which there is a widely opened space, as far away as possible from any surrounding obstacles. As a result, a plan of the least problematic pick-and-place operation can be determined.
However, our previous proposal considers only the relationship between an object and the hand of a robot at the central gripping plane on selecting the grasp orientation relative to the object. Therefore, the robot hand may interfere with the object in the determined grasp position and orientation. It was thus required that after determining the grasp position and orientation, the judgment was made as to whether or not the actual grasping was feasible on the basis of the place positions and geometric data of the robot hand and the object. Since the avoidance of interference between the object and the hand was carried out in the trial-and-error manner, the plan could not efficiently be determined.
In certain operation plans, the hand cannot transfer from its initial state directly to its goal state. Instead, the hand intermediately places the grasped object and regrasps the placed object in order to transfer it to the goal position. In the past, there was known a technique for planning regrasping which had previously provided data of the list position of the hand on gripping and regrasping location (see P. Tournassoud, T. Lozano-Perez, "Regrasping", Proc. IEEE International Conference on Robotics and Automation, pp.1924-1928, Vol.3, 1987).
In the actual grasp plan, the feasible grasp position varies depending on changes in the environment. Thus, a previously determined grasp position may be infeasible in a subsequent operation. This means that the grasp plan itself fails. Particularly, when the regrasping operation is to be used, the environment around the hand and the object will vary for each pick-and-place operation. This results in an increase in the probability that the previously determined grasp position will be incorrect.
If several pick-and-place operations were to be performed, the regrasping locations could not be selected automatically.
In an operation plan including the regrasping step, an object must be first picked up and then placed on a table and transferred to the next state after the grasp position and orientation has been changed in the hand. In the normal case, however, there are a great number of the object's positions and orientations through which the object can be placed on the table. This will provide a corresponding number of state transition paths from the initial state to the goal state. If an estimation is made with respect to work efficiency in all the combinations, the processing time period becomes huge.
Our previous proposal has a further problem in that it provides an inefficient grasp plan or plans excessive regrasps, in the case that the objects have rotational symmetries.